In the mounting of electrical circuit devices, such as a ceramic carrier for an integrated circuit silicon chip, on a ceramic substrate having thin or thick film integrated circuits formed thereon, the chip carrier generally is mounted on the substrate so that the chip carrier is slightly elevated with respect to the substrate, such as on the order of 5-20 mils. The mounting of the chip carrier on the substrate is accomplished by bonding leads of the chip carrier extending down the sides thereof, to contact pads on the substrate. It then is necessary to cover circuitry on the substrate beneath the chip carrier with a void-free protective coating, such as a self-curing RTV encapsulating material comprised essentially of silicone rubber. If the undersurface of the chip carrier facing the substrate also is provided with thin or thick film integrated circuits, it then becomes essential that this surface also be covered with a void-free protective coating of the encapsulating material. The void free coatings prevent moisture films from forming on the substrate and/or the chip carrier, the moisture films being undesirable because they cause leakage currents between adjacent circuit elements of the substrate and/or the chip carrier, and/or corrosion of the circuit elements.
In a known process for encapsulating one or more integrated circuit silicon chips mounted directly on a substrate, without a chip carrier, encapsulating material is flowed between the chips and the substrate, and over the substrate-chip assembly. The encapsulated substrate-chip assembly then is placed in a vacuum for one minute, to draw air bubbles out of the RTV encapsulating material. The encapsulated assembly then is air-dried and partially cured for two hours, after which the RTV encapsulating material is heat-cured for three hours at 120.degree. C.
An encapsulating method as above-described has worked satisfactorily in the past since the silicon chips, which did not require carriers, generally were relatively small in size, not exceeding 0.100 inches square, and generally were spaced from the substrate on the order of only up to 5 mils. When this method is used with chip carriers, however, which have dimensions up to on the order of 0.700 inches square, and which are spaced from the substrate up to on the order of 20 mills, the RTV encapsulating material tends to develop voids under the chip carrier, producing surface areas of the substrate and/or the chip carrier which are not covered by encapsulating material. This is undesirable because at least one mil of encapsulating material over any circuit elements on these surfaces is considered essential to provide proper protection of the circuit elements against the subsequent formation of moisture films which could produce detrimental leakage currents between the circuit elements and/or corrosion of the circuit elements as above-described.
In this connection, it also has been suggested that more reliable results can be achieved if the opposed surfaces of the substrate and the chip carrier initially are coated with thin layers of encapsulating material with an air gap between the coated surfaces. Otherwise, such as where the entire space between the chip carrier and the substrate is filled with encapsulating material, as the encapsulating material shrinks during curing it tends to pull away from the chip carrier and substrate surfaces, exposing them to moisture films as noted above.
A problem has existed, however, in coating the opposed surfaces of the chip carrier and the substrate with the thin layers of encapsulating material separated by an air gap, and various systems have been proposed for this purpose. For example, the encapsulating material has been blown into the space between the chip carrier and the substrate by a jet stream at high velocity, causing a thin film of encapsulating material to be deposited on the chip carrier and substrate opposed surfaces, with the excess encapsulating material therebetween being blown away by the jet stream. The chip carrier and the substrate, with the space therebetween filled with encapsulating material, also have been subjected to a spinning operation, causing the excess encapsulating material to be thrown out of the space between the chip carrier and the substrate by centrifugal force.
In conjunction with the foregoing coating processes utilizing blowing of the RTV encapsulating material between the substrate and the chip carrier, or spinning of the substrate-chip carrier assembly, it also has been proposed that the substrate-chip carrier assembly initially be cleaned with a solvent, such as xylene, for activating curing of the RTV encapsulating material. The substrate-chip carrier assembly then in blow-dried and the RTV encapsulating material is applied between the substrate and chip carrier as above described, and about the sides of the chip carrier. The partially encapsulated substrate-chip carrier assembly then is placed in a vacuum for one minute, after which the entire substrate-chip carrier assembly is flow-coated with the RTV encapsulating material. The encapsulated substrate-chip carrier assembly then is successively dried in a nitrogen atmosphere for 6 hours, and in air for 3 hours (spinning process) or 40 hours (blowing process), and finally heat-cured at 120.degree. for 2 hours (spinning process) or 3 hours (blowing process). However, neither of these proposed processes has yet been proven entirely satisfactory for the intended purpose.
Accordingly, a primary purpose of this invention is to provide a new and improved, relatively simple method of encapsulating a chip carrier mounted on a substrate, without the formation of voids in the encapsulating material adjacent opposed surfaces of the chip carrier and the substrate.